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Forum » SpaceEngine » Science and Astronomy Discussions » Science and Astronomy Questions
Science and Astronomy Questions
WatsisnameDate: Wednesday, 07.08.2013, 22:26 | Message # 166
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Right, if there are several in the same galaxy (e.g. the Magellanic Clouds), then we know that within said galaxy they all lie at roughly the same distance and thus the relationship of period to absolute magnitude is really true for them. This is exactly the trick that Ms. Leavitt used when she discovered them roughly a century ago. However, the relation alone doesn't tell you the distance unless you also know the distance of at least one Cepheid via another method, so one had to be found relatively nearby such that we could use the Parallax Method. Then, knowing both the distances of some nearby Cepheids, and also knowing the period-luminosity relation, we can measure distances much farther than we could with Parallax alone (Parallax is useful only out to a few thousand light years), and go out as far as we can reliably resolve Cepheids (thankfully they can be quite bright, and we can see them even in other nearby galaxies). They are thus often called 'standard candles' in astronomy.

To reach even farther than Cepheids, we use another set of standard candles -- Type Ia supernovae. And then there is Hubble's Law, and other useful relationships. The principle of using multiple methods, relationships, and standard candles across many scales is the Cosmic Distance Ladder, and it allows us to accurately measure distances from the Moon to the edge of the universe. smile





 
midtskogenDate: Thursday, 08.08.2013, 07:20 | Message # 167
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Quote (Watsisname)
To reach even farther than Cepheids, we use another set of standard candles -- Type Ia supernovae. And then there is Hubble's Law, and other useful relationships. The principle of using multiple methods, relationships, and standard candles across many scales is the Cosmic Distance Ladder, and it allows us to accurately measure distances from the Moon to the edge of the universe.

As the "ladder" indicates, the calibration of the larger scales depends on a chain of relationships, which through the history of astronomy has made it difficult to get accurate figures, but as science progresses, precision improves, and perhaps today, allowing a not too strict idea of accuracy, we finally can say that we can measure the larger distances pretty accurately. There will still be improvements, though.





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WatsisnameDate: Friday, 09.08.2013, 02:06 | Message # 168
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Oh yes, astronomical distance measurements come with astronomical uncertainties. tongue I'd venture that over Gpc distances an error of ten or twenty percent is probably typical. Personally I think that's still pretty good, and is incredible progress considering the state of knowledge only a few decades or a century ago. There will continue to be improvements of course, particularly with more accurate / longer range parallax measurements which forms the base of the chain. But there's a lot that can be refined further.




 
HarbingerDawnDate: Friday, 09.08.2013, 03:10 | Message # 169
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Quote (Watsisname)
particularly with more accurate / longer range parallax measurements which forms the base of the chain

Can't wait until Gaia launches smile





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SalvoDate: Tuesday, 13.08.2013, 12:48 | Message # 170
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What cause exactly the greenhouse effect? The density or the composition of an atmosphere?

Because for example on the abitable exoplanets catalog they calculate ESA assuming that every exoplanet have earth like atmosphere, but that's quite unlikely, planets with greater mass will probably have denser atmosphere... (and we don't even know what's the probability that they forms an nitrogen-oxygen atmosphere like earth)





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Edited by Salvo - Tuesday, 13.08.2013, 12:49
 
HarbingerDawnDate: Tuesday, 13.08.2013, 14:30 | Message # 171
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Greenhouse effect is caused by composition of the atmosphere.




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themohawkninjaDate: Tuesday, 13.08.2013, 17:45 | Message # 172
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Two questions regarding galaxies.

1. What part of the galaxy does the Right Ascension, and Declination coordinates refer to? The middle?, a corner of the galaxy?

2. Excluding the Milky Way, what nearby (that is to say, we can get a sharp hi-res image of) galaxy has the most supernova recorded in it?
 
HarbingerDawnDate: Tuesday, 13.08.2013, 18:15 | Message # 173
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Quote (themohawkninja)
What part of the galaxy does the Right Ascension, and Declination coordinates refer to? The middle?, a corner of the galaxy?

It has nothing to do with the galaxy, it's relative to Earth's equator. Also, the galaxy does not have corners, as it is disk shaped.





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themohawkninjaDate: Tuesday, 13.08.2013, 18:39 | Message # 174
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I understand that it's disk shaped, I was thinking more along the lines of "squaring the circle" per se. Those numbers are what allows astronomers to know where to point their telescopes, right? If not, then what does allow astronomers to know where to point their telescopes, and again, where on the galaxy would those exact coordinates point to?
 
HarbingerDawnDate: Tuesday, 13.08.2013, 18:42 | Message # 175
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Quote (themohawkninja)
Those numbers are what allows astronomers to know where to point their telescopes, right?

Yes





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themohawkninjaDate: Tuesday, 13.08.2013, 18:51 | Message # 176
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So then, if you were to take the precise Right Ascension, and Declination coordinates of a spiral galaxy and input them into a large observatory telescope, and look through the telescope, what part of the spiral galaxy would be at the center of the screen?
 
midtskogenDate: Tuesday, 13.08.2013, 19:05 | Message # 177
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Quote (themohawkninja)
So then, if you were to take the precise Right Ascension, and Declination coordinates of a spiral galaxy and input them into a large observatory telescope, and look through the telescope, what part of the spiral galaxy would be at the center of the screen?

I'm pretty sure that the coordinates of galaxies are assigned for their centres, but by which method the centre is determined I don't know, nor do I know if there is a strict convention.





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HarbingerDawnDate: Tuesday, 13.08.2013, 19:06 | Message # 178
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The middle, and that applies to virtually every object. It's like describing the geographic coordinates of a country, you just pick someplace roughly in the middle.




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midtskogenDate: Tuesday, 13.08.2013, 19:18 | Message # 179
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Quote (HarbingerDawn)
It's like describing the geographic coordinates of a country, you just pick someplace roughly in the middle.


There are common ways to find the geographical centre of a country not based on some rough estimate. But does something similar exist for galaxies? Since the border of a galaxy is not well defined, we can't use a geographic centre as for countries. Ideally it should be the true gravitational centre, but that is too hard to establish. It could be its brightest point, or the centre based on a weighed sum of its (visible) light.





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HarbingerDawnDate: Tuesday, 13.08.2013, 20:19 | Message # 180
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Quote (midtskogen)
There are common ways to find the geographical centre of a country not based on some rough estimate.

Of course, but the point that I was trying to make is that for an object which occupies a large area, it doesn't matter what point you pick to identify its position as long as you consistently use that point, and for useful reasons that point ought to be roughly in the center of the object, or (in the case of galaxies and nebulae which are poorly characterized) centered on the brightest part of the object.





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